1. Technical Field of the Invention
This invention broadly relates to a capsule comprised of a chemical composition surrounded by an exterior protective coating. The invention more particularly relates to the material employed to produce the exterior protective coating which surrounds the chemical composition contained in the capsule. The invention further relates to the release of the composition from the capsule.
2. Description of the Prior Art and Problems Solved
The prior art has long dealt with the problem of causing, or of creating the conditions to cause, a specified chemical composition to react in a specified environment or in a specified location at a specified time or over a specified period of time. This problem ordinarily stems from the need to initiate some chemical action in some desirable location prior to the start of, during or subsequent to the completion of some other action, and/or to cause the chemical action associated with a specified chemical composition to proceed over a desirable period of time. The related problems of controlling the time and controlling the place of reaction of a composition, as well as the solutions thereto, have been variously referred to in the art as delayed release, continuous release and controlled release and are referred to as such herein.
The prior art has addressed the problems referred to above and has developed various methods of controlling the introduction of an active chemical composition into reactive contact with an environment. The solutions to the problems have necessarily involved preventing reactive contact of the active chemical composition with an environment followed by enabling reactive contact to occur at some later time. One of the methods developed features the steps of preventing the mentioned reactive contact by completely covering a small quantity of the active chemical composition with a material to produce a capsule containing the composition followed by introducing a number of such capsules into a designated environment wherein the composition is released from the capsule to enable reactive contact to thereby produce a desired chemical action.
The mode of release of the active chemical composition into reactive contact with the environment as contemplated by the prior art has proceeded along various different mechanisms. Such release mechanisms range from external crushing forces applied to the confining material, internal rupture and bursting forces applied to the confining material, disintegration of the confining material and diffusion or permeation of liquid through the confining material. Accordingly, the release mechanisms employed have featured the use of capsules which are crushed to rapidly release the enclosed chemical composition; capsules which rupture, or burst, to rapidly release the enclosed chemical composition; capsules which dissolve or disintegrate to rapidly release the enclosed composition; and capsules which do not rupture, but rely on diffusion or permeation, to gradually release the enclosed composition.
Chemical compositions known to have been involved in the controlled release thereof from a capsule have included medicines, pesticides, herbicides, cosmetics, laundry products, pigments, polymerization initiators, cross linking agents and viscosity reducing agents. A delayed and/or a continuous release of such compositions from enclosing capsules can offer the advantage of maximizing the effectiveness of the compositions or of minimizing or eliminating undesirable chemical or physical results or both.
The essential differences between the various controlled release methods of the prior art have resided in the nature of the material employed to form the exterior protective coating and the specific mechanism of the controlled release of the chemical composition from the confining material. Examples of such prior art methods are disclosed in various United States Patents including U.S. Pat. No. 4,756,844 (WALLES I), U.S. Pat. No. 4,741,401 (WALLES II), U.S. Pat. No. 4,923,753 (WALLES III), U.S. Pat. No. 4,919,209 (KING) and U.S. Pat. No. 5,373,901 (NORMAN et al).
The prior art patents referred to above all disclose capsules containing active chemical compositions and the methods, that is, the mechanisms, of releasing the compositions from the capsules to enable the encapsulated chemicals to react with a composition in contact with the exterior of the capsules. Walles I illustrates its invention in terms of an encapsulated bleach as a laundry detergent additive. Walles II, King and Norman et al illustrate their inventions in terms of an encapsulated breaker for a fracturing fluid in a subterranean formation. Walles III illustrates its invention in terms of an encapsulated acid. The chemical compositions disclosed range from those which treat water based liquids to those which treat oil based liquids. Furthermore, the release mechanisms disclosed and/or referred to range from capsules which are crushed to rapidly release the enclosed agent; capsules which rupture, or burst, to rapidly release the enclosed agent; capsules which do not rupture, but rely on diffusion or permeation, to gradually release the enclosed agent; and capsules which dissolve or disintegrate to rapidly release the enclosed agent.
An important, if not the essential, feature of a method of controlling the release of a composition from a capsule is the timing of the release. This feature can be conveniently illustrated in connection with the release of a chemical from a capsule to reduce the viscosity of a fracturing fluid subsequent to the performance of a fracturing treatment of a subterranean formation. Reducing the viscosity of a fracturing fluid is referred to in the art as “breaking” and the viscosity reducing chemical is referred to as a “breaker.” For purposes well known in the fracturing art, it is desirable that a fracturing fluid have a high viscosity during the performance of the fracturing process, but a low viscosity subsequent to the performance of the process. In this regard, a desired high viscosity is preferably to be observed in the vicinity of the subterranean formation to be treated and a desired low viscosity is also preferably to be observed in the vicinity of the subterranean formation, wherein the high viscosity condition occurs prior to and during the performance of the fracturing treatment and the low viscosity condition occurs subsequent to the performance of the fracturing treatment.
Breakers, upon reactive contact with the fracturing fluid, can effectively produce the desired reduction of viscosity of the fluid, but the timing of the break, that is, when the break occurs, is of critical importance. Breakers, upon reactive contact with the fracturing fluid, begin to reduce the viscosity of the fluid. Accordingly, the problem to be solved is how to intimately mix the breaker with the fracturing fluid to enable essential reactive contact with the fluid, while at the same time delaying the time of reactive contact. This problem has been addressed and solved by placing the breaker in an enclosed volume called a capsule, the wall of which is a membrane which shields the fluid from the breaker to prevent reactive contact. The capsule containing the breaker is then intimately mixed with the high viscosity fluid, and the membrane, by one of the mentioned mechanisms, functions to release the breaker to enable reactive contact between the breaker and the fluid at some future time. The breaking of fracturing fluids and the use of encapsulated breakers to perform the task are subjects discussed in Norman et al, U.S. Pat. No. 5,373,901.
An important property of the material of construction of a capsule membrane is its ability to resist a caustic environment, either acid or base, exhibited by the chemical enclosed in, and thus in contact with, the interior of the capsule and by the chemical in contact with the exterior of the capsule. The sensitivity of the material of construction to a caustic environment is the subject of the disclosure of U.S. Pat. No. 4,923,753 (WALLES III) referred to above.
It is important from an operational point of view to be able to place chemical compositions of various natures, including caustic chemicals, in a capsule and to retain and maintain them therein shielded from the exterior environment until some desirable future time. A capsule thus constructed of a membrane material which will function to hold and maintain various diverse chemicals, such as, organic and inorganic caustics, salts and oxidizers, and which will also function to release the chemicals at some desirable time is desired by the art. Accordingly, the material of construction of the capsule coating is the principal issue addressed in this invention.